Six you already know
You know exactly what these do. Almost none of them still shows what it means.
A code with no key and no chart
A cipher has a key. A resistor band has a chart. These have neither. You learned them the way you learn slang: by watching what happened when other people pressed them.
Which makes them the most fragile codes on this site, and the toughest. The day everyone forgets what a floppy disk was, the icon still works, because it stopped meaning disk and started meaning save.
Some are built to need no key at all: the skull, the running figure on a fire exit, the red bar through a circle. Those aim at the opposite of a cipher, readable by a stranger who shares no language with you. And some fail: a microphone means record in one app and talk to the assistant in another. A symbol only means something inside an agreed system. Outside it, it means whatever the receiver guesses.
Write a message in app icons
Take the first letter of what the thing is called. Play is P, mic is M, wifi is W. Anyone who can name the icons can read it on sight, with no key at all.
That trick is called an acrophonic alphabet, and it is how the first one was built: the Phoenician letter for house, beth, became our B.
J, O and Y have no obvious icon, so they are simply missing: the same honest gap as I and J sharing a cell in a Polybius square.
Polybius squaresThe same trick out loud
Alfa, Bravo, Charlie does exactly this with sound instead of pictures: a whole word standing in for its first letter, so it survives a bad radio.
The NATO alphabet Back to signals